package com.lion.common.base.utils;

public class SnowflakeIdGenerator {
    // 起始时间戳（2020-01-01 00:00:00）
    private final static long START_TIMESTAMP = 1577836800000L;
    
    // 每一部分占用的位数
    private final static long SEQUENCE_BIT = 12;   // 序列号占用的位数
    private final static long MACHINE_BIT = 5;      // 机器标识占用的位数
    private final static long DATACENTER_BIT = 5;   // 数据中心占用的位数
    
    // 每一部分的最大值
    private final static long MAX_DATACENTER_NUM = ~(-1L << DATACENTER_BIT);
    private final static long MAX_MACHINE_NUM = ~(-1L << MACHINE_BIT);
    private final static long MAX_SEQUENCE = ~(-1L << SEQUENCE_BIT);
    
    // 每一部分向左的位移
    private final static long MACHINE_LEFT = SEQUENCE_BIT;
    private final static long DATACENTER_LEFT = SEQUENCE_BIT + MACHINE_BIT;
    private final static long TIMESTAMP_LEFT = DATACENTER_LEFT + DATACENTER_BIT;
    
    private long datacenterId;  // 数据中心ID
    private long machineId;     // 机器ID
    private long sequence = 0L;  // 序列号
    private long lastTimestamp = -1L; // 上一次时间戳

    public SnowflakeIdGenerator(long datacenterId, long machineId) {
        if (datacenterId > MAX_DATACENTER_NUM || datacenterId < 0) {
            throw new IllegalArgumentException("Datacenter ID can't be greater than " + MAX_DATACENTER_NUM + " or less than 0");
        }
        if (machineId > MAX_MACHINE_NUM || machineId < 0) {
            throw new IllegalArgumentException("Machine ID can't be greater than " + MAX_MACHINE_NUM + " or less than 0");
        }
        this.datacenterId = datacenterId;
        this.machineId = machineId;
    }
    
    public synchronized long nextId() {
        long currentTimestamp = getCurrentTimestamp();
        
        if (currentTimestamp < lastTimestamp) {
            throw new RuntimeException("Clock moved backwards. Refusing to generate id for " + (lastTimestamp - currentTimestamp) + " milliseconds");
        }
        
        if (currentTimestamp == lastTimestamp) {
            // 相同毫秒内，序列号自增
            sequence = (sequence + 1) & MAX_SEQUENCE;
            // 同一毫秒的序列数已经达到最大
            if (sequence == 0L) {
                currentTimestamp = getNextMill();
            }
        } else {
            // 不同毫秒内，序列号置为0
            sequence = 0L;
        }
        
        lastTimestamp = currentTimestamp;
        
        // 时间戳部分 | 数据中心部分 | 机器标识部分 | 序列号部分
        return ((currentTimestamp - START_TIMESTAMP) << TIMESTAMP_LEFT)
                | (datacenterId << DATACENTER_LEFT)
                | (machineId << MACHINE_LEFT)
                | sequence;
    }
    
    private long getNextMill() {
        long mill = getCurrentTimestamp();
        while (mill <= lastTimestamp) {
            mill = getCurrentTimestamp();
        }
        return mill;
    }
    
    private long getCurrentTimestamp() {
        return System.currentTimeMillis();
    }


    // 测试方法
    public static void main(String[] args) {
        // 创建ID生成器（参数为数据中心ID和机器ID）
        SnowflakeIdGenerator idGenerator = new SnowflakeIdGenerator(1, 1);

        // 生成10个ID
        for (int i = 0; i < 10; i++) {
            long id = idGenerator.nextId();
            System.out.println(id);
            System.out.println(String.valueOf(id).length());
            System.out.println("-----------------");
        }
    }
}